Usually, planets move from west to east against the background stars. Sometimes for a while the planets appear to move from east to west. This is due to the visual effects of the changing relative positions of Earth and the planet in question.
Retrograde motion is caused when the Earth moves past the outer planet on its faster inside track. Consider in this diagram that Earth is on the inside track. It orbits faster than does an outer planet, let us say Jupiter, so undertakes (overtakes ‘underneath') it. In the illustration notice how the line of sight lines up against the background stars. Initially Jupiter moves eastward going from the brown star in the background to the red star. It slows down as it approaches the orange star. Very quickly it turns westward – retrograde – heading back to the yellow star and further to the green star when viewed from position 5. Thereafter, it resumes an eastward motion past the blue star towards the purple star.
A model as shown in this diagram gives a practical demonstration of this retrograde phenomenon. The pointer shows the line of sight from the the Earth (where it is fastened to the inner disk) to the outer planet represented by the yellow dot - for instance Jupiter. By turning the inner disk clockwise, it can be seen how the pointer moves in one direction except when the Earth moves between Jupiter and the Sun, when it moves in the opposite direction.
A model can be built according the guidelines in this diagram.
The Starwaders Retrograde Model can be purchased for R150.
(Production halted due to lack of demand. Please contact me for further details. nevyoung AT starwaders.com)
Send an email to firstname.lastname@example.org with your contact and postal address details. State which models you are buying and the total price.
An EFT transfer can be made into the account details below. Add R30 for post and packaging. For cash deposits add another R15 to the price.
You are welcome to make arrangements to collect the model on a COD basis. Ask for address and directions in your email.